A conventional transmission for a powered vehicle can include a gearbox, i.e., gears, synchronizers, dog clutches, clutch plates and reaction plates, a number of automatically selectable gears, planetary gear sets, hubs, pistons, shafts, and other housings. The clutches can be stationary brakes/clutches or rotating clutches. The transmission can have an internal shaft which rotates one or more clutches or shafts in the transmission.
The number of clutches and gear schemes can be used to achieve different gear ratios. Each gear ratio can define a range in which the transmission operates. A higher numerical gear ratio, for example, can be achieved at a lower transmission output speed. This can be important if a vehicle is heavily loaded or requires higher torque to ascend an elevation, for instance. Alternatively, a lower numerical gear ratio can be achieved at a higher transmission output speed, particularly when a vehicle is travelling at higher speeds on a highway. The lower numerical gear ratio can help increase fuel efficiency at these speeds.
To achieve different gear ratios, gear schemes are configured in which brakes, clutches, shafts, and gearsets are packaged in a transmission. The gear scheme can include one or more planetary gearsets. A planetary gearset can include a sun gear, a ring gear, and a carrier. One or more of the sun gear, ring gear, and carrier can be input or output of the planetary gearset. The manner in which torque is transmitted through the clutches and gearsets determines the different gear ratios for a given transmission.
In some arrangements, however, a gear step between shifts precludes the shift from being done automatically. Instead, a vehicle operator may have to manually shift the transmission from one gear ratio to the next with the throttle released because the gear step is too large. Gear step is defined as the ratio between gear ratios. In transmissions that include a torque converter and lockup clutch, it may also be difficult to apply the lockup clutch due to the increased gear step.
There is a need for a seven-speed, automatic transmission having a gear arrangement for automatically selecting gears and which the gearbox is configured for minimizing gear step between gear ratios for forward and reverse ranges.